Problem Solved: How to Fix a String of Christmas Lights

We've all been there: You get your holiday lights out of storage to start decorating the house, only to find a string or two won't light up. Repairing the lights isn't always easy but, with a little determination and know-how from our partner site PopularMechanics.com, it is possible.

At some terrible moment in early- to mid-December, we can almost hear the collective sigh of dismay as good people around this land untangle and plug in strings of bad mini lights. Above the din we also hear a few expletives, because the mass-produced sets can be pricey. Your impulse to fix them is noble, but the task should not be undertaken lightly (sorry, unavoidable pun), because the strands contain scores, if not hundreds, of tiny incandescent bulbs. Which one is the culprit? Or are there several burnouts?

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Regardless, don't give up. With a few handy tools and a modicum of holiday spirit, mini lights can be repaired.

First step: diagnosis. You need to figure out if it's actually a bad bulb causing the malfunction or something else. Keep in mind that smaller sets are wired in series — that is, the electrical current must pass through each individual bulb in order to complete the circuit and illuminate the set. Larger strings often feature two or more series circuits wired in parallel, which explains why sometimes just a section of the strand won't light up. Often, simply replacing a bad bulb will fix the entire set or at least a section of it.

But locating the faulty bulb can be tricky. You'll need either a set of basic electrician's tools or the LightKeeper Pro ($19.95; lightkeeperpro.com), a dedicated tool that combines a voltage detector, a bulb remover, bulb and fuse testers, and a shunt repairer. More on that later.

If you're lucky, simply plugging in the mini lights will reveal a bad bulb. "Bad" may just mean it needs to be seated more firmly in its socket. In any case, you can remove and test a bulb using the LightKeeper Pro or a multimeter. If you install a new one, make sure it has the proper voltage rating or you risk damaging the whole set.

Fixing a string of lights isn't always that simple. All or part of it may be dark because of a broken filament or a sketchy shunt. If a bulb's filament breaks, the shunt is supposed to redirect current through the base of the bulb, maintaining the electrical circuit. You can fix a faulty shunt with the LightKeeper Pro. Simply plug in the strand, remove a bulb that's in or near the darkened section, and insert the tool into the bulb's socket. Pulling the trigger activates a piezo circuit, which sends a high-energy pulse through the set. After about 20 pulses, any faulty shunt should be activated.

If you're still having trouble locating where the circuit is broken, try using a voltage detector. There's one built into the LightKeeper Pro, or you can use an electrician's model. Still nothing? Check the fuse located behind a small sliding panel on the male end of the plug. If it's fried (you can check it with the LightKeeper Pro), be sure to replace it with a new one of the correct rating, which should be indicated on the plug. And if all else fails, buying a new string may be irksome, but at least it'll keep some small part of your holidays bright.